"Tomorrow Is Yesterday" is a first-season episode of the original science fiction television series Star Trek. It is episode #19, production #21, first broadcast on 26 January 1967, repeated 13 July 1967, and was remastered in 2006 for syndication broadcast on 5 May 2007. The teleplay was written by D.C. Fontana and directed by Michael O'Herlihy.

In the plot, the crew of the USS Enterprise travels back to 1969 Earth and must correct damage they caused to the timeline.

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On stardate 3113.2, the Federation starship USS Enterprise is thrown back in time to Earth during the 1960s by the effects of a high-gravity "black star". The Enterprise ends up in Earth's upper atmosphere, and is picked up as a UFO on military radar.

A U.S Air ForceF-104 interceptor piloted by Captain John Christopher (Roger Perry), is scrambled to identify the craft. The pilot, surprised to see the strange craft above him, is ordered to stop the Enterprise from escaping. Captain Kirk uses a tractor beam on the jet, which accidentally tears the plane apart. Kirk orders the pilot to be transported aboard the Enterprise to save him.

The man is at first confused by his new surroundings, then amazed by what the future holds, and impressed and awed when he discovers that Enterprise is one of 12 such starships under the authority of a combined service and that the computer calls Kirk "Dear", having been overhauled on the female-dominated planet Cygnet XIV (the technicians there thought the computer needed a personality).

Fearing Christopher could disrupt the timeline if returned to Earth after glimpsing the future, Kirk decides Christopher must stay with the Enterprise. After Science Officer Spock later discovers that the pilot's own as-yet-unborn son will play an important role in a future space mission to Saturn, Kirk realizes he must return Christopher to Earth without any knowledge of Enterprise or other future events.

After learning of the existence of film taken of Enterprise by Christopher's wing-cameras, Kirk and Lt. Sulu beam down to the airbase to recover it. Upon obtaining the sensitive computer-tapes, Kirk is captured and disarmed by an Air Policeman.

When Spock attempts to contact Kirk, via communicator, the policeman accidentally activates an emergency-signal and he is immediately beamed aboard. With yet another abducted and confused native to deal with, they confine him to the transporter-room.

Spock, Sulu, and Captain Christopher, knowledgeable of the base's layout, beam down to find and recover Kirk. After Kirk's guards are subdued, Christopher grabs one of their guns and demands to be left behind. Spock, out of the room, has himself transported to a position behind Christopher, where he disables him with a Vulcannerve pinch.

After they return to the ship, Spock and Chief Engineer Scott inform Kirk of a possible escape method by slingshotting around the Sun to break away and return to their time. The maneuver is risky, since even a small miscalculation could destroy the ship, or make them miss their own era.

Kirk okays the maneuver, and time on board the Enterprise moves backwards. Capt. Christopher is beamed back to his fighter jet at the instant he first encountered Enterprise, preventing any evidence of the ship being produced, which somehow erases his memory of his time on the Enterprise,and the sighting is written off as just another UFO. The Air Policeman is also returned to his own time, just moments before he first stumbles upon Kirk and Sulu, and somehow his memory of his Enterprise visit is erased, too. Enterprise then successfully returns to the 23rd Century, in the right time.

Zack Handlen of The A.V. Club gave the episode a 'B-' rating, describing it as "so-so" and "passable".[1]

In contrast, Jamahl Epsicokhan of Jammers Reviews rates "Tomorrow is Yesterday" as "a brilliantly fascinating story, beginning with its exciting opening shots of the Enterprise flying through Earth's sky, and continuing through an adventure where the crew must remove all traces of their presence in the past."[2] Jamahl notes that this episode was "... a pioneer time-travel outing for Star Trek, and a great one at that, beginning a tradition of storytelling open to limitless possibilities."[3] Jamahl gives this episode a 4 star excellent rating.

The episode was originally conceived as part two to an earlier episode, "The Naked Time"; when the ending to that episode was revised, "Tomorrow is Yesterday" was reworked as a stand-alone story.[4]

Associate producer Robert Justman devised the original idea for the story, and it was handed to Dorothy Fontana to create a teleplay. Justman received neither credit nor payment for doing so, whereas Roddenberry's agent charged the studio up to $3000 for his own stories and rewrites.[5]

The method of time travel used in this episode was again used by the crew in the motion picture Star Trek IV: The Voyage Home, this time to travel back to the year 1984. The same method was utilized a third time to return the Enterprise to the 23nd century.

The second issue of IDW Publishing's comic book series Assignment: Earth (a continuation of the episode of the same name, drawing on the episode's status as a failed backdoor pilot for a spin-off television series) shows the protagonists, Gary Seven and his assistant Roberta Lincoln, becoming peripherally involved in the events of "Tomorrow is Yesterday", acting on their own to prevent the Enterprise '​s presence from affecting history. Due to peculiarities of time travel, the Enterprise crewmembers have not yet met Seven and Lincoln at this point, but it is Seven's and Lincoln's second encounter with them (though they are careful to avoid direct contact, so as not to alter the proper course of events).